This invention relates to an inorganic EL panel, and more particularly, to a full color EL panel comprising light emitting layers for producing three colors RGB.
In the recent years, active research works have been made on thin-film EL devices as small or large-size, lightweight flat panel displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a dual insulated structure using thin-film insulating layers 2 and 4 as shown in FIG. 3. In FIG. 3, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrode-bearing substrate 1. On the first, insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to the environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primary colors of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescence center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primary colors of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. With respect to blue, in particular, a relatively high luminance is achieved using SrS:Ce. However, its luminance is yet short as the blue color for full color display, and its chromaticity is shifted toward the green side. There is a need to have a better blue light emitting layer.
To solve the above and other problems, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shingaku Giho (Communications Society Technical Report), EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from a low luminance and especially, difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. In particular, thioaluminate base phosphors are quite difficult to control their composition.
All the EL spectra of the aforementioned blue, green and red EL phosphor thin films are broad. When they are used in a full-color EL panel, RGB necessary as the panel must be cut out of the EL spectra of the EL phosphor thin films using filters. Use of filters complicates the manufacture process and, still worse, brings about a lowering of luminance. When RGB are taken out through filters, the luminance of blue, green and red EL phosphor thin films is lost by 10 to 50% so that the luminance is reduced below the practically acceptable level.
RGB filters for full color display are generally RGB filters which are formed in their own pixel pattern on a glass substrate, separately from the substrate having formed thereon phosphor films for RGB. The glass substrate is set in alignment with the phosphor substrate to construct a full color panel. However, this method requires two micropatterned substrates and the manufacturing process is complex and expensive and thus impractical.
To solve the above-discussed problems, there is a desire to have a panel capable of red, green and blue light emissions of good color purity and a high luminance without a need for an RGB filter in the formed of a patterned glass substrate.
Most of prior art EL phosphor thin films are deposited at relatively high temperatures. When these phosphor films are patterned by photo-lithography so as to provide the three primary colors, the process encounters extraordinary difficulty and is not practical. In particular, those phosphor thin films capable of emitting light of a high luminance and a high color purity are deposited at higher temperatures. It is extremely difficult with currently available material systems to pattern such phosphor thin films all by photo-lithography to construct a full color display of a high luminance and high definition.
An object of the invention is to provide an EL panel, especially a full color EL panel, comprising phosphor thin films for producing light of an improved color purity without a need for a RGB patterned filter substrate.
The present invention provides an EL panel for providing a display of three colors, comprising at least EL phosphor thin films of two types for emitting green and blue colors, and a red light emitting portion, wherein the EL phosphor thin films of two types for emitting green and blue colors comprise an alkaline earth sulfide or oxide.
In a preferred embodiment, only the red light emitting portion has a color conversion layer. More preferably, the red light emitting portion includes a phosphor thin film for emitting electroluminescent light from an output surface and the color conversion layer disposed on the output surface.
In another preferred embodiment, the red light emitting portion is constructed by combining the EL phosphor thin film for emitting green or blue color with a color conversion layer.
In a preferred embodiment, the red light emitting portion includes an EL phosphor thin film composed primarily of ZnS:Mn.
In a preferred embodiment, the EL phosphor thin film for emitting green color comprises as a matrix material an alkaline earth thiogallate which may contain oxygen, and the EL phosphor thin film for emitting blue color comprises as a matrix material an alkaline earth thioaluminate which may contain oxygen. More preferably, the EL phosphor thin film for emitting blue color comprises as a matrix material barium thioaluminate. Further preferably, the EL phosphor thin film for emitting green color comprises as a matrix material strontium thiogallate. Most often, both the phosphor thin films contain Eu as a luminescence center.
In a preferred embodiment, when expressed in the CIE color purity coordinates (x, y), the green color emitted has coordinates of x less than 0.3 and y greater than 0.6 and the blue color emitted has coordinates of x less than 0.2 and y less than 0.2.
In a further preferred embodiment, the EL phosphor thin films of two types are represented by the compositional formula:
AxByOzSw:R 
wherein A is at least one element selected from the group consisting of Mg, Ca, Sr, Ba and rare earth elements; B is at least one element selected from the group consisting of Al, Ga and In; x is a number from 0 to 5, y is a number from 0 to 15, z is a number from 0 to 30, w is a number from 0 to 30; and R is an element serving as a luminescence center.
In a still further preferred embodiment, each of the EL phosphor thin films of two types for emitting green and blue colors comprises an oxysulfide which contains oxygen and sulfur elements such that the molar ratio of O/(S+O) is in the range from 0.01 to 0.85.